Laser ablation, or laser evaporation deposition (LEDE) is a known technique for growing thin films on a substrate. The technique has been used to deposit high temperature superconductor materials as thin films on a variety of substrates.
An apparatus for performing laser ablation deposition, representative of the prior art, is seen in FIG. 1. The apparatus includes a vacuum chamber 1, a thermomolecular pump 2 which imparts a vacuum in the chamber 1, and a gas manifold. A pulsed laser beam 3 from a KrF source (not shown) is focused by a cylindrical lens 4, through a quartz window 5, onto a target pellet 6 made of superconductor material, such as YBa.sub.2 Cu.sub.3 O.sub.7.
The beam 3 striking the pellet 6 generates a plume 7 containing the target material. The material is then deposited on a substrate 8 mounted on a heater 9. Typical of the prior art, the target pellet 6 is caused to rotate during ablation, as shown by the directional arrow.
In the pulsed laser ablation process with a smooth flat target surface, the initial plume direction is normal to the surface and nearly independent of the laser beam incident angle. For the deposition of thin films to take place properly, the film substrate must be placed so as to intercept the plume, and the optimum placement is usually found to be on the target normal.
For multipulse depositions, the laser cannot pass through the film or it will ablate it away, negating the whole effort. Consequently, the laser beam incident angle must be off normal.
In depositing high temperature superconductors such as YBCO by laser ablation, it has been found that texturing of the target surface occurs as the laser ablates away material with repeated pulses, and a typical film may need thousands of pulses. This texturing creates microscopic finger-like projections pointing back along the laser beam. These cause a steering of the ablation plume away from the target normal and toward the laser beam. The more pronounced these projections become, the more steering they produce. Both film thickness per pulse and film composition (at least for YBCO composite targets) are affected, causing severe degradation of film quality and repeatability unless steps are taken to control texturing.
It has been suggested that allowing buildup of the texturing increases the generation of large ablation particles which can degrade the film. Thus, most prior art laser ablation techniques have employed target rotation or translation as a means of reducing the effect of texturing.
A technique that I had previously proposed used a laser beam that scans up and down across the disk's diameter at 1/10 the rotation rate.
However, this technique produces a plume which moves up and down as the laser beam scans. A moving plume is troublesome for ablation processes where there is a need to keep the plume in a fixed location relative to the chamber.
Another laser/optical problem in laser ablation deposition is the need for high spatial uniformity of the laser beam incident on the target in order to get uniform film composition. This requirement is typically met with existing excimer lasers by masking off all but the center of the laser beam, thus wasting 50 to 70% of the laser pulse energy.
More sophisticated laser resonator optics can reduce the amount of masking needed, but they reduce the total energy extractable from a given laser, so that little gain in overall efficiency is achieved. The laser operating cost is probably the most significant economic factor for large scale use of the laser ablation deposition process, so efficient utilization of the laser light is imperative.
Another problem experienced by laser ablation processes is laser beam entrance window fogging or contamination by the ablation plume. This can cause a 50% loss in laser energy delivered to the target after a few thousand pulses, thus requiring that the windows be removed and cleaned after a few films have been deposited. This is clearly a production bottleneck for any large scale use of the process, but even in research use, it costs time and money to remove and clean the windows. Moreover, window cleaning can lead to contamination of the vacuum chamber with air and moisture. Also, exact process conditions which produce a given research sample cannot be guaranteed since significant fogging can occur in a single film run.